FIG. 20 illustrates a conventional sorting method. In the conventional sorting method, a wet specific-gravity sorting utilizing differences in specific gravity between types of plastics is performed on a mixed plastic piece group including a mixture of plastic pieces of a plurality of types. With this, the plastic pieces are sorted and collected into polypropylene (PP) plastic pieces having a specific gravity of 1.0 or less, acrylonitrile butadiene (ABS) plastic pieces, polystyrene (PS) plastic pieces, and high specific-gravity PP plastic pieces (PP plastic pieces having a high specific gravity due to glass fiber or the like added thereto, which are represented as “heavy PP” in FIG. 20) each having a specific gravity of from 1.0 to 1.1, and high specific-gravity plastic pieces having a specific gravity of 1.1 or more.
The PP plastic pieces other than the sorted high specific-gravity PP plastic pieces are subjected to washing and foreign matter removal so as to obtain recycled PP plastic pieces.
The ABS plastic pieces, the PS plastic pieces, and the high specific-gravity PP plastic pieces each having substantially the same specific gravity are subjected to first stage electrostatic sorting by utilizing characteristics of plastics, that is, differences in triboelectric series of types of plastics. With this, the plastic pieces of those types are separated and collected as positively charged ABS plastic pieces, negatively charged PS plastic pieces, and negatively charged high specific-gravity PP plastic pieces. Of those, a mixed plastic piece group of the PS plastic pieces and the high specific-gravity PP plastic pieces is further subjected to second stage electrostatic sorting other than the first stage electrostatic sorting. With this, positively charged PS plastic pieces and negatively charged high specific-gravity PP plastic pieces are sorted and collected (for example, refer to Non Patent Literature 1, Patent Literature 1, and Patent Literature 2).